386 ADAPTATIONS TO MEDIA AND SUBSTRATES 



until it aims horizontally, there being a special provision for this in the 

 form of a temporary lower lid. Still others have kept the eyes aimed more 

 nearly upward and have given them protection, from over-stimulation, 

 by means of expansible pupillary opercula. 



Such an operculum is most nearly a group-character in the batoids 

 (i e., rays in the broad sense) .These elasmobranchs (but not Squatina) 

 lack the eyelids (Fig. 131b, p. 382) which characterize the bottom-loving 

 sharks (Galeorhinidce) , but they can nevertheless retract and 'close' their 

 eyes at times to shield them from strong light (p. 452). The eyes are 

 relatively small, as they are in all upward-lookers, which have not the 

 need for a large pupil that a lateral-eyed fish has. A ray's eyes are little 

 more than half the size of those of a shark of equal size. The pupillary 

 operculum ordinarily has a smooth margin (e.g., Torpedo, Trygon, 

 Myliobatis) , but in Raja it is serrated so that, on full expansion, it re- 

 duces the pupil to a crescentic series of stenopaic apertures (Fig. 65b, 

 p. 158). The operculum of Torpedo is small, but it can cut the slender, 

 horizontally oblong pupil quite in two. In the mantas or devil-fishes 

 (Mobulidae) and eagle-rays (Myliobatidae) , the eyes aim not upward but 

 laterally, due to the presence between them of a pronounced ridge of 

 head material. The mantas lack a pupillary operculum, though one is 

 present in Myliobatis. 



A mutual exclusiveness of pupillary opercula and turreted orbits is also 

 suggested by the situation in teleosts. The operculum varies from small 

 (in the star-gazer, Uranoscopus scaber, where it is dentate — see Fig. 

 65d) to large (flounders), and is remarkably developed in the armored 

 catfish Plecostomus (Fig. 65e, f, g). The bulk of the flounders are in- 

 cluded in the families Bothidae (left-handed) and Pleuronectidae (right- 

 handed flounders) . In the bothids, the eyes tend to lie fairly flat in the 

 head, and an operculum (Fig. 65c) is the rule; but the eyes of pleuro- 

 nectids, by and large, lack opercula and can be elevated hydraulically, and 

 swivelled about in the horizontal plane by a special slip of the superior 

 oblique muscle. Some pleuronectids, however (e.g., Platichthys flesus), 

 do have opercula. The ocular turrets of flatfishes make it possible for 

 them to see even while the body is sifted over with sand for concealment. 

 A lateral aim of the eyes obviates any handicapping of the horizontal 

 vision of the animal when it rises from the bottom to become pelagic 

 for the nonce, as do the mantas, eagle-rays, and many flatfishes. The 

 binocular vision, now forward, now upward, of the turret-eyed flounders 

 gives these fishes what has been called an 'intelligent' look. 



